A battery is one of the essential parts of a boat; without a battery, a boat will not start, and it will not power your electricals onboard. Even after taking proper care of them, they will fail in the end, intimating you to replace them. So, how often should a boat battery be replaced?
As a general rule, replacing your boat battery once every 4-5 years is recommended, but if it is well maintained and passed load & voltage tests, you can use it for few more years. A well-maintained battery could last 10 years, and if it is not well maintained, it couldn’t last 2 years.
However, before replacing any battery, regardless of the battery type (flooded or deep cycle), it needs to be load and voltage tested. If it passes the test and gives positive results, you can still use it for another year or more. We will discuss how to test the battery later in the post.
If a battery is properly maintained, not allowed to fully discharge, stored indoors in freezing and hot weather, not allowed to fully charge continuously, and water added when needed. A basic wet cell lead-acid battery and others can last up to 10 years. If it is maintained well enough, you can replace it once every 7-10 years.
You will definitely hear from many people saying that they will replace the boat battery every 3 or 4 years regardless of whether it is in good condition or bad condition and use it for other purposes. If you are more concerned about the money, it’s good to avoid that saying or rule.
That narrows you down to replacing the battery after testing its performance with a load tester and multimeter. We will now see how to test the battery performance DIY or at the battery shop.
How to test a boat battery?
A voltage test is a good and quick reference but not the proper way to check batteries. A bad battery can still put out 12.6 volts but not for very long at all. It needs to be load tested to get precise answers. Testing the battery will clearly give all the insights, and from that, you can decide whether to continue using it or replace it.
If it comes back up to 100% & passes the load test and the voltage test, then it’s good, if not replace. This way, your decision to replace is based on some kind of logic. First will look at how to load test the battery and then voltage checking.
NOTE – Temperatures are crucial while testing the battery. So, the best time to take the reading is early in the morning before the sun hits you or in the evening as the sun is setting. Don’t test it in very hot or cold weather.
Load testing a boat battery
For load testing a battery, you need a battery load tester. If you are doing it occasionally, once every 3 or 6 months, you don’t need to buy a load tester because almost any store that sells marine or RV or other batteries will usually load tests for free. So, you can take your battery and test it there itself.
But ensure to charge your battery fully before doing any tests on that battery. If you want to test the battery often, you can buy one load tester for less than $50. Check this ANCLE load battery tester (linked to Amazon), which can support large amperage (CCA) values, especially for load testing a battery.
That being said, we will now see how to load test a battery by yourself with a battery load tester.
- Charge your battery fully (100%) and let it sit overnight unplugged, and then only perform the test. Otherwise, it will give bad (inconclusive) results due to the charge fluctuations (low charge).
- Connect the negative wire (black wire) of the load tester to the battery’s negative terminal and the positive wire (red wire) of the load tester to the battery’s positive terminal.
- Select one-half of the CCA value on the load tester (you can see the CCA value on the battery, if you can’t find it, you can contact them).
- The test is performed for 15 seconds (the load is applied for 15 seconds). Check the voltage values and CCA values throughout the test for 15 seconds.
- While during the test, if the CCA values on the load tester go below 80% of one-half of the battery’s CCA value, and if the voltage values go below 12.4V, it’s time to replace your boat battery.
Below is a table showing the voltage values of a 12V battery (all types). They determine the voltage levels at different DOD levels (source).
| State of Charge | Flooded Lead Acid battery voltage (in volts) | Gel battery voltage (in volts) | AGM battery voltage (in volts) |
| 100% | 12.70+ | 12.85+ | 12.80+ |
| 75% | 12.40 | 12.65 | 12.60 |
| 50% | 12.20 | 12.35 | 12.30 |
| 25% | 12.00 | 12.00 | 12.00 |
| 0% | 11.80 | 11.80 | 11.80 |
After completing the whole test, wait for 1 or 2 minutes and perform the test again (multiple times) to get the best results out of it. Then finally, decide upon those values. Some load tester will directly tell whether it is good or bad. Like ANCLE load battery tester (linked to Amazon), it displays on its display whether the battery is good to use or not.
If you want to (load) test for your battery quite often (once every 2 weeks or once every month), it is highly recommended to buy a load tester, and you can get one for less than $50.
Check this helpful video on testing the boat batteries with a load tester to get an illustrated view of the above explanation or procedure.
Voltage testing a boat battery
Checking the battery voltage is another way to decide whether to replace your battery or not. You can get a good multimeter for less than $20, and you can test it all by yourself. The procedure is as follows.
- Charge your battery fully (100%) and let it sit overnight unplugged, and then only perform the test. Otherwise, it will give bad (inconclusive) results due to the charge fluctuations (low charge).
- Connect the negative wire (black wire) of the multimeter to the battery’s negative terminal and the positive wire (red wire) of the multimeter to the battery’s positive terminal.
- If those values go below 12.4 volts, you need to replace the battery.
Now, you might be wondering why we need to do the load test if the voltage test is that simple. You need to do both the test to confirm whether the battery is dead or working. A bad battery can still put out 12.6 volts but not for very long at all. It needs to be load tested to get precise answers.
So, it’s good to carry a small multimeter in the boat. Check this Cernova digital multimeter (linked to Amazon) if you want a multimeter, and it does your job better than most of the premium ones.
Check this short and helpful video on how to test your Boat Battery Voltage with a multimeter to get an illustrated view of the above explanation or procedure.
How to increase your boat’s battery life?
There are some simple ways to improve your boat’s battery life. By doing them, your battery could last 7-10 years in total. We will now see how to increase your boat’s battery life.
Don’t deeply discharge a boat battery
Discharging a boat battery deeply oftentimes (75% or 50% or less) will decrease the battery’s total number of cycles dramatically. When a boat battery has been subjected to deep discharge, the amount of electricity that has been discharged is actually 1.5 to 2.0 times as great as the battery’s rated capacity.
One more thing to consider is a battery that has been deeply discharged takes a longer time to charge it fully (100%) than a normally discharged battery (not deeply). The occasional dip to 75% or 50% DOD (Depth of Discharge) is fine, but you need to try to recharge it up with 100% ASAP.
Generally, 50% is the max recommended DOD by most all AGM & flooded battery manufacturers. So, make sure you won’t get below that quite often. The less deep discharge you regularly go, the more number cycles you will get, which implies more life (years).
Fully charge and disconnect them during a long term storage
Fully charging a boat battery (100%) before storing it away for any long-term storage or winter is crucial. Any battery will self-discharge over time, and you can’t stop it from happening that. If it is not fully charged, it will hurt the battery’s life eventually, and it could die quickly.
And another thing to consider here is disconnecting the battery during the winter or long-term storage is crucial. If a battery is not disconnected, all the electrical devices onboard will slowly discharge the battery over time, even if they are turned off. So, disconnecting the boat battery during long-term storage is essential.
So, pull them for winter storage or long-term storage, and keep them in the garage on a wood platform or a rubber platform for most of the winter. Or just disconnect them and leave them on a boat but charge them fully before leaving them. And just before using them, check the voltage, and if it is still less than 12.4 volts, replace them.
Don’t leave a battery on a charger continuously
Charging a battery is essential, but overcharging it will kill the battery over time. This is one of the main reasons why the flooded battery will bulge outside. Overcharging a battery causes excessive gassing — the electrolyte gets hot, and both hydrogen and oxygen gas are generated.
On older vented batteries, the electrolyte could cook away, leaving the plates exposed and ruining them. On sealed batteries, the buildup of gases could cause the battery to burst. Even if you leave the charger connected continuously, at a small 2 amps, the battery eventually will die.
So, don’t put your battery on the trickle charge or any charger continuously. Many chargers are not as “smart” as they claim. Charge to full, then disconnect is often safer. Float or maintenance chargers such as a Battery Tender keep the battery charged without the risk of damage.
Don’t keep a battery in hot places
The temperature will affect the boat battery most because high temperatures will not only be a loss of battery’s capacity but also leaks or ruptures the battery as well. Store your batteries at room temperature or below. The recommended storage temperature for most batteries is 59 °F (15 °C) (source).
As temperatures increase, especially over 100 °F (38 °C), so does your batteries’ internal discharge. If a new fully charged battery is left sitting 24 hours a day at 110 °F for a month, it would most likely not start an engine. Cold weather isn’t nearly as hard on batteries as hot weather.
The chemistry just does not make the same amount of energy in bad conditions. A deeply discharged battery can freeze solid in sub-zero weather. So, store your batteries well during extreme winter and hot summers to avoid any problems.
Fully charge a battery as often as possible
While using your boat every day, the battery discharges. Fully charging the battery will improve the number of cycles. However, the number of cycles won’t increase beyond the manufactures max cycle count. Using a battery at lower DOD (50% and less) will damage the battery and puts more load than a fully charged battery.
It will eventually degrade the battery’s life if it is used while deeply discharged. Weekly or biweekly, getting your battery up to 100% as a minimum guideline will dramatically improve cycle life. So, charge it as often as possible, but ensure to charge it fully weekly or biweekly.
It’s OK to use a battery between 50% or 60% DOD and 80% or 90% DOD while cruising. But then try to get a good full 100% overnight charge at least once or twice a week.
Check electrolyte regularly on flooded batteries
Whenever you are working with a battery, you’ll want to take some safety precautions. Not only can the battery give you quite a shock, but it can also become dangerous if not handled properly. Ensure you have gloves and safety goggles and disconnect the battery before you start messing around with it.
First, clean the battery with a brush and a mixture of baking soda and water near the terminals and top. Then remove the plastic tops covering the cell ports with the help of a screwdriver or any. Once the covers have been removed, carefully clean away any dirt that may have built up underneath.
Now check the electrolyte levels inside each cell and add only the distilled water, not any other water form, if electrolytes are not up to the level. A common rule of thumb is to add enough water to cover the electrodes or plates. Then cover the cell ports and charge it fully and then test whether it is working properly oo not.
Key takeaways
On average, it is good to replace your boat battery every 4-5 years based on the load and voltage test results. If it passes the load and voltage test, you can use it for another year or more. So, how often to replace your boat battery depends on how well it is maintained.
However, before replacing any battery, regardless of the battery type (flooded or deep cycle), it needs to be load and voltage tested. If it passes the test and gives positive results, you can still use it for another year or more.
If a battery is properly maintained, not allowed to fully discharge, stored indoors in freezing and hot weather, not allowed to fully charge continuously, and water added when needed. A basic wet cell lead-acid battery and others can last up to 10 years. If it is maintained well enough, you can replace it once every 7-10 years.